This research project is engaged in elucidating the enzymic mechanisms involved in the assimilatory reduction of nitrate to ammonia. The significance of this process is manifest in that the various oxidized inorganic forms of nitrogen provide the ultimate source of nitrogen for all life. Nitrate assimilation is a two-step process: the two-electron reduction of nitrate to nitrite, followed by the six-electron reduction of nitrite to ammonia. The enzymes catalyzing these reactions, nitrate reductase and nitrite reductase respectively, are adaptively formed in Neurospora crassa in the presence of either nitrate or nitrite; their synthesis is repressed by ammonia. Nitrate reductase transfers electrons from NADPH to nitrate via its component functional groups according to the following scheme: NADPH yields ((-SH) yields FAD yields cytochrome b557 yields Mo) yields NO minus over 3. Nitrite reductase catalyzes the stoichiometric reduction of nitrite to ammonia using three equivalents of reduced pyridine nucleotide via the sequence: NAD(P)H yieds ((FAD, -SH) yields siroheme) yields NO minus over 2. Recent studies in this laboratory on purified N. crassa nitrite reductase (mol. wt. 290,000) reveal it is a dimer of 140,000 subunit mol. wt. Current objectives involve further characterization of its complement of prosthetic groups and their involvement in the various electron transport functions of this enzyme complex, the ultimate goal being a concise description of the relationship between structure and function in this nitrite reductase.